The invention relates to a device including a DC voltage source whose negative electrode is connected to a cathode and circuit means for periodically imposing a positive potential on the cathode.
In sputtering installations in which a target of a cathode is sputtered by impinging ions, so that the sputtered target particles are deposited on a substrate directly or after preceding chemical combination with other particles, arcing is frequently observed. This arcing occurs as a rule between cathode and anode; however, arcing can also occur between the electrodes and other parts of the installation. Arcing occurs particularly frequently during reactive sputtering of metal oxides or metal nitrides. The reason for this can be found therein that the generation on the target surface of layers which are more or less well-insulating cannot be avoided. These layers, in turn, form small capacitors with the target surface. Since these layers stand exposed to the plasma, they are charged and lastly form an electric field strength of such magnitude that a breakdown occurs causing an arc discharge between cathode and anode. Herefrom result spot destructions of the target and, consequently, layer defects on the substrate.
In order to avoid arcing of this type, it has already been suggested to bring a DC magnet cathode connected to a DC power source with the aid of a matched additional circuit periodically for short periods of time to a positive potential wherein the frequency of the periodic reversal of the poles can be set as a function of the layer to be deposited (German Patent Application 42 02 425.0). The pole reversal is carried out with the aid of four switches of which two can establish a connection between a voltage source and the cathode of the installation, and two a connection between this voltage source and the anode of this installation. As switches can therein be used thyristors which, however, require relatively complicated triggering.
Furthermore is known a process and a device for the careful coating of electrically conducting objects by means of a plasma in which the electrical energy is supplied with periodically repeated DC pulses (German Patent No. 37 00 633). This method is not suited for coating electrically nonconducting substrates.
The invention is therefore based on the task of simplifying the reversal of poles of an electrode in a DC sputtering installation, in which also electrically nonconducting substrates can be coated or etched.
This task is solved by periodically connecting a second DC power supply to the cathode.
The advantage achieved with the invention resides in particular therein that the substrate-independent internal arcing is suppressed. The power supply feeding the plasma, i.e. supplying it with energy, is not pulsed but rather is a DC power supply. A power supply of this type is simple and inexpensive since the expensive switch is omitted which is necessary with pulse operation. The pulsed power source utilized in xe2x80x9cinternal arcingxe2x80x9d does not supply energy to the plasma. Rather, due to its opposite polarity, it leads to the interruption of the coating process.